It is said in Scripture and Tradition that at the time of the birth or announcement of every Messenger of God, a star or a sign appears in the heavens.
Nimrod was warned of the star that told of the coming of
Abraham. The soothsayers warned Pharaoh of the star in the heavens that
foretold the coming of Moses. The Magi informed Herod of the new star that guided
them to the throne of the "spiritual king," Jesus. The same legend is
told of Buddha, Zoroaster, Muhammad and Krishna.
What were the signs in the heavens during the appearance of
the Báb and Bahá'u'lláh? The holy Scriptures of all faiths had spoken of Twin-Revelations
that would appear at the "time of the end." Now that the Báb and Bahá'u'lláh
had appeared, fulfilling these prophecies, what were the signs in the
heavens? Signs, not for one, but for two
Messengers of God, Who would appear almost simultaneously?
Some of us know the story of the great comet of 1843 which foreshadowed
the coming of the Báb.
Sir James Jeans, late British astronomer and mathematician,
stated in his book ‘Through Space and Time’, "oddly enough, many of the
most conspicuous appearances of comets seem to have coincided with, or perhaps
just anticipated, important events in history." 
One of the most unique stories of a comet is that told of
the period during which the Báb and Bahá'u'lláh were engaging correspondence,
and during which the Báb was preparing His followers for the appearance of Bahá'u'lláh.
This story was told in the stars as well as on the earth.
In 1845 a comet appeared soon after the one in 1843. It was
called Biela's Comet. It seemed to be an ordinary comet, in a year in which
some 300 comets had appeared, and it had appeared many times before in the
past. In 1846 it was still visible. At this period in its history, it became
one of the unique comets of all time. It
was now entering into the last dramatic moments of its life.
The Encyclopedia Americana gives the following account of
"It was found again late in November 1845, and in the following
month an observation was made of one of the most remarkable phenomena in astronomical
records, the division of the comet. It put forth no tail while this alteration
was going on. Professor Challis, using the Northumberland 1846, was inclined to
distrust his eyes or his glass when he beheld two comets where but one had been
before. He would call it, he said, a binary (twin) comet if such a thing had ever
been heard of before. His observations were soon verified, however." 
History shows that there had been other binary (twin comets)
but Biela's was one of the most unusual. Sir James Jeans wrote of this same
comet, saying: "The most interesting story is that of Biela's comet which
broke in two while under observation in 1846."
The comet then disappeared.
It returned in August, 1852. This
was the very month and the very year in which Bahá'u'lláh was cast into an
underground prison in Tihran.
This was the year 1269 of the calendar of Islam. It was also exactly the ninth year after the
Báb's Declaration to Mulla Husayn in the year 1260. The Báb had prophesied:
"Ere nine years have elapsed" the Promised One of all ages and
religions will come.
It was but a few weeks later, in that same prison, that Bahá'u'lláh
's Mission began.
August, 1852, was the hour of the reappearance of the comet,
the comet that had split in two to become twin-comets. Strangely enough, at the time of the
reappearance of the twin-comets in 1852, one half had receded far into the
background. The other half, though in a parallel
orbit, now dominated the scene. Just so
had the Báb, the Herald of the Baha'i Faith, passed into history, and the One
Whose coming He had foretold, Baha'u'llah, had assumed His Mission.
The Americana record of this astronomical event states: "Late
in August, 1852, the larger portion again came into view; and three weeks later
the smaller one, now much fainter than its former companion was seen about
1,500,000 miles in the lead." Sir James Jeans confirms this: "Six
years later , when the comet's orbit again brought it near to the sun,
two pieces were observed to be one and a half million miles apart."
Of that hour when the twin-comets rode the skies above His
prison, Bahá'u'lláh has written: "I was but a man like others, asleep upon
my couch, when lo, the breezes of the All Glorious [God] were wafted over Me
and taught Me the knowledge of all that hath been. This is not from Me, but from One Who is
Almighty and All-knowing." 
Following this twin-appearance, Sir James Jeans states,
"neither of them has been seen in cometary form, but the place where they
ought to be is occupied by a swarm of millions of meteors, known as the Andromedid
meteors. Occasionally these meet the
earth in its orbit, and make a grand meteoric display."
The two comets were no longer separate comets, but were
mingled in one great shower of light, just as the Faith of the Báb and Bahá'u'lláh
are not separate but one in the light which they shed upon the earth.
- William Sears (‘Release the Sun’)
 Sir James Jeans, ‘Through Space and Time, pp.102, 154
 Encyclopedia Americana vol. III, 1944 ed., p.691
 Shoghi Effendi, ‘The Promised Day is Come’, pp. 40-41